Production of metal powders



c. L. MANU-:LL 2,182,567

Filed Nov. 27. 1936 P -fa ,4A/0055 v c4 777/0055 PRODUCTION oF METAL PowDERs ///PECT CU/P/E/VT 50u/PCE /waw/A/ Dec, 5, 1939.

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f7-nf@ l INVENToR (Hx/H55 f/GH MAM/7E@ BY movil MIM @um ATTORNEY Patented Dec.. 5, 1939 UNITED STATES 2,182,567 PRODUCTION F METAL OWDERS charles Leigh Munten, Manhasset, N. Y., assignor to Hardy Metallurgical Company, New York, N. Y., a. corporation of Delaware pplicatlon November 27, 1936, Serial No. 112,928

schim@ Y vThis .invention vrelates to the productionv of metal powders and particularly to the production of metal powder particles which have cores and coatings of dissimilar metals." The invention contemplates especially the production of coppercoated lead powders, ire., plated metal powder particles on which the plating is of a metal standing'lower thanthe metal of the core in an electromotive force series of metals, and the inhibition of crystal growth of the lead powders. A

Any element in such a series tends to plate out of solution on a metal of the series which is less electropositive, i, e., upon a metal which stands higher in the series. -For example, if metallic iron is introduced into a solution of .copper sulfate the iron replaces the copper in the solution with the result that metallic copper is precipitated. A replacement reaction of this character'is characterized industrially as a process of cementation andrthe precipitate is known as a cement.

v The cementation processes have been employed heretofore for the production of plated metal powder particles having coatings of a metal which is'lower in an electromotive series than the metal of the cores.- Patent No. 2,033,240, granted March 10, 1936, to Charles Hardy, to produce copper-coated` lead powders for use in the manufacture of heat-resistant bearings 'by stirring lead powder in a solution of a copper salt, say copper acetate. Copper-coated lead powders made by a cementation process as described in the aforementioned patent are of satisfactory quality. There is, however, a marked disadvantage of such a cementetion process in that it makes as a by-product a salt of the less electropositive metal. Thus lead acetate is formed when lead powder is stirred into a solution of copper acetate. The by-product, if discarded, represents a loss of 'valuable material. On the other hand, the cost Vof recovering the by-product, especially if the recovery operation is conducted on a small scale, maybe more than the value of the by-product.

As a result of my investigation I have discovered that it is possible to produce copper plated lead powder particleswithout making a salt of the lesselectropositive metal (lead) asa byproduct. This isv accomplished by`\i conducting a cementation reaction in an acetate` electrolyte in an electrolytic cell, and passing current through 4the solution in the cellso 'as to precipitate as a.

powder the less electropositive metal' (lead) which thus forms nuclei upon which the more electropositive metal (copper) precipitates as a Thus, it is proposed in United States (Cl. 20d-1) I coating. In this way, the concentration of the less electroposltive metal (lead) in the solution is kept from increasing, andthe cores for the plated metal powders are formed in situ. 'Ihe production of plated metal powders is thus greatly 6 simplified and it becomes unnecessary to introduce the lead into the process in powder form and also unnecessary either to discard a valuble solution of a salt of the lead or to subject it to a separate recovery operation. A

The density of the current introduced into the solution while cementation is taking place should be sufliciently high to cause the lead to precipitate as a powder, rather than in' massive form. j u My invention will be more completely under- 15 stood in the light of the following detailed description of my invention with particular reference to the production of copper-coated lead powders where it is particularly useful, because of the diiliculties ordinarily encountered inattempt- 20- ing to produce a lead powder from a cheap solution of a lead salt by .electrolytic' means. Efforts to obtain satisfactory lead` powders by subjecting lead perchlorate electrolytes, lead acetate electrolytes, lead persulphate electrolytes, lead chloride 25V electrolytes and alkaline lead cyanide electrolytes to electrolysis have not been markedly successful. Lead powders satisfactory for some purposes may be obtained by subjectingxthese solutions of these relatively cheap lead salts to electrolysis, but 'the 30 powders thus produced are generally unsatisfactory as a base for making copper-plated lead powders or the like. The lead tends to precipitate as long acicular crystals or as at plates. Particles ofsuch shape are less satisfactorythan the shapes which approach the cube or sphere. There is, moreover, a tendency for the formation of crystals which are too large to beemployed successfully in powder metallurgy, where a great deal depends upon using ne powders in order to obtain a dense mass of accurate configuration with a minimum amount of heat and pressure applied to the powders. Thirdly, the lead precipitated from such electrolytes either vcontains a substantial quantity of oxidized leadas itisformedy l; or becomes excessively oxidizedbefore it can be l washed and freed of the excess electrolyte. 'A fourth and from a commercial standpoint the greatest diiiiculty is that lead'powde'rs produced from the aforementioned cheap electrolytes tend to mat into an 'impervious mass which is difficult if notimpossible towashvor to separate into its discrete particles. f f y.

.'However, I have-discovered that-when asmallamount of a soluble copper salt' is present in an 55 aqueous acetate electrolyte, satisfactory coppercoated lead powders are formed at relatively low current density, and the tendency for the forma- `tion of acicular orplaty powders or powders which form impervious mats is not observed. Moreover, the lead particles are substantially evenly coated with copper and are not oxidized to an appreciable extent.

Copper salts in aqueous acetate baths in which lead powders are being formed electrolytically apparently act to restrain the crystalvgrowth of thelead particles. It has been known previously that certain organic materials, principally colloids such as glue, may act as crystal-growth retarders or restraining agents in the electrorefining or deposition of metals. However, insofar as I am aware, it has not been known heretofore that the salt of one metal will act torestrain the crystal growth of another and less electro-positive metal during the electro-deposition of the latter.

The following example illustrates the restraining influence exerted by copper salts in acetate electrolytes upon crystal growth of lead powders during electrodeposition of the latter and also illustrates the production of copper-coated lead powders in an electrolytic cell by a process which eliminates the addition of separately formed lead powders to a copper salt solution and the subsequent discard or treatment of a lead salt solution to extract its lead content either in massive form or vas lead powder to be returned,

An electrolytic cell was employed in which anodes and cathodes were hung vertically. The cathodes were of sheet nickel, although copper, lead, or other suitable materials may be employed. If contamination of the product with a small amount of iron can be tolerated, cheap sheet iron.

can be used for cathodes.

The anodes employed were rods of copper and of desilverized lead hung side by side between the cathodes in the proportions indicated.

A means for removing the copper-coated lead powder from the bottom of the cell lwas pro; vided. The powder, after removal from the cell, was washed with distilled water to free it of entrained salts. lIhe wash water entrained by the powder was replaced by alcohol and the powder was then dried. High drying temperatures should be avoided because of the low melting point of the lead.

ExAnPLn Electrolute Grams per liter Lead acetate i 94.8 Copper acetate 49.9 Acetic acid 15.0 Sodium' chloride-; 2.0

l Anodes 4 copper rods 1 leadrod of same size and surface as each copper rod Anode current density .'amps. per sq. ft 8.8 Cathode current density do 17.6 Current efliciency percent-- 22.2

Power consumed, kilowatt hours per pound of metal deposited 1.5

der produced was of small and uniform particle size and apparently uniformly coated. The tendency for depletion of copper in the electrolyte was not very pronounced.

A series of tests conducted along the lines of thel foregoing example showed that there is a tendency for copper to deposit out of the electrolyte faster than it is replenished from the soluble .copper anodes and at a rate out of proportion with that at which the lead is deposited. 'I'his tendency may be overcome by employing one or` tamination of the deposit with insoluble lead4 chloride.

If there is a. tendency for the copper content of the electrolyte to become depleted, this is to some extent compensated by an increase of dissolution of lead at the anode, so that in part the depleted copper content is replaced by lead. However, as the copper content of the electrolyte decreases in the cell, the acid content generally increases, so that in effect hydrogen ion is replacing the copper ion in the bath.

(2) Increasing the ratio of exposed copper anode surface to exposed lead anode surface in the bath. This has a tendency to increase the `corrosion rate of the. copper as compared with the corrosion rate of lead at the anode. Control by this means is, however, limited by the amount of copper which can be tolerated in the coppercoated powder. As the copper surface at the anode is increased and the lead surface decreased the proportion of copper'in the powder deposited increases. To produce powder containing no more than copper the ratio of copper surface `to lead surface at the anodes preferably should not exceed 4 to 1. y

-(3) Adjusting of the current density. In general, the higher the current density at the anode the greater is lthe tendency for dissolution of copper to occurf 'Ihis effect is accompanied, however, by an increase in the rate at which copper is deposited from the solution. Moreover, there is a limit to the current density that may be employed in any particular case which is set by the particle size desired. Increased current densities apparently operate to increase particle size.

4. Offsetting the tendency of the' copper content of the electrolyte to decrease by adding soluble copper. to the electrolyte from an outside source. As already indicated, the acid content of the electrolyte increases as the copper content decreases. 'I'his increased acid content may be neutralized by employing basic salts of copper. 'Ihe salts most satisfactory from a commercial standpoint are the carbonates, hydroxides, and oxides of copper. 'I'hese salts are cheap and do not introduce a foreign basic radical into the electrolyte. This fourth method of control is preferable in that it 'is more flexible than the electrolyte from the cell' either periodically or continuously, and to add to the withdrawn electrolyte a basic salt of copper, whereby the acid content of the electrolyte is reduced and the copper'content is replenished. Thereafter the electrolyte is separated from any excess of the basic copper salt and returned to\the cell.

It is possi- 76 ble to conduct the reaction between the basic salt of copper and the acid in theelectrolyte within the cell, but preferable'to conduct itelsewhere to avoid contaminating the deposit with unconsumed basic salt `or disturbingl the action of the y cell by the evolution vof reaction products such as carbon dioxide generated by the action of acid upon copper' carbonate.

'I'he following practice was found to be pref,

`arable for the `production of a copper-coated lead "powderl'containing about 60% 011.40% Pb by l weightandirofa particle size such that substan-A tially--all Vof .the powder will pass through a screen having 200 meshes to the linear inch (Tye` ler scale) ,f/ An aqueous electrolyte isy prepared 'at theine` ception ofthe operation by dissolving about 100 grams of lead acetate, 50 grams of coppi?? acetate, 15 grams of acetic acid and at least 2 grams of sodium chloridein each liter of water. -Any compound which will give chloride ions in solution may be used in'K place of sodium chloride.

Direct current is passed through the electrolyte in a cell equipped with soluble lead and soluble copper anodes or an anode containing both lead and copper arranged so that the exposed surface area of the copper within the electrolyte is aboutl fo'ur times that of the exposed surface area of .l

the lead in the electrolyte. Rods of copper and of desilverized lead hung vertically in the cell have proved to be satisfactory. i

The cathode from which the plated powder migrates may be of any suitable conductive mate? rial which does not tend to corrode with deleterious contamination of the electrolyte. Copper and nickel 'are preferred materials. Iron may be' used if contamination of the electrolyte can be tolerated.

The form of the cell should be such that anodes and cathodes may be easily inspected and, if necessary, scraped to remove deposits.` A cell of the type that is employed'to produce copper powders by electrodeposition is preferable. How` 4 ever, almost any cell may be' employed provided that it permits inspection and occasional scraping of anode and cathode, and is provided with means for withdrawing the powder formed before it has had an` opportunity to oxidize excessively.

-A reaction chamber should be provided with means such as a pump for circulating the electrolyte from the cell to the reaction chamber and back again. The reaction chamber should be provided with an agitator so that the basic salt of copper, preferably an oxide or carbonate `in nely divided form,- may be caused to react quickly and thoroughly with free acid in the elec- -trolyte thereby increasing the copper content and decreasing the free acid content of the latter. The amount of copper salt added should be such as to maintain the copper and acid content of the electrolyte in the neighborhood of its original composition.

As a safeguard against the contamination of the electrolyte in the cell with an undissolvedexcess of the basic salt or of insoluble impurities therein, the electrolyte from the .reaction Y chamber should be sent throughl an appropriate mtengsettung device"qr the like before it, 1s re- 1 turnedv to the cell. g e

In the event that a settling chamber or nlter for trapping V soliclsufrom the outlet ofthe-reaction chamber, the circulation through.

zthe cellshould be gentle or a settling chamber or lter should be employed at )its outlet towardtheelectro 'as non gmt m u the reaction chamber, to avoid losing metal powupon particles of lead, theimprovement' which' comprises passing a direct .current through an 'aqueous acetateelectrolytecontaining acetic acid and relatively large proportions of copper' and of lead'in solution in an' electrolyticcell containing va cathode,v a lead anode, and "a copper Lanode,`

maintaining the cathode current density in excess of that at which lead deposits from the electrolyte in massive form so that the lead deposits from the electrolyte as substantially non-adherent fine particles Aand said particles become plated with copper from the solution and controlling the ratio of copper to lead in the resulting coppercoated lead particles `by varying the' ratio of exposed copper anode surface in the solution to exposed lead anode surface in the solution.

A2. In the manufacture of copper-coated lead particles in which'copper is plated from a solution upon particles of lead, the improvement which comprises passing a direct current through an aqueous acetate electrolyte containing acetic acid and relatively large proportions of copper and of lead in solution anda relatively small proportion of a water-soluble chloride in solution in an` elec-4 trolytic cell containing a cathode, a copper anode and a lead anode, maintaining the cathode current density in excess of that at which lead deposits from the electrolyte in massive form,

whereby the lead deposits as substantially nonadherent iine particles and said particles become plated with copper from the-electrolyte, reacting the resulting acid spent electrolyte with a basic Salt of copper and thereafter returning the re'- sieilnting replenishedfelectrolyte to the electrolytic c 3. In the manufacture of copper-coated lead powder in which copper is plated from a solution upon particles,- of lead, the improvement which comprises passing a direct current through yan aqueous' acetate solution containing acetic acid and relatively large proportions of copper and of lead in solution and a relatively small proportion of awater-soluble chloride in solution in an electrolytic cell containing a cathode,a lead anode and a copper anode, maintaining the cathode current density in excess of that at which lead deposits from the solution in massive form so that the lead deposits as substantially 'non-adherentne particles and said particles become plated with copper'from'the solution, andsmaintaining the anode current density substantially less than the cathode current density. y 4. In Vthe manufacture of copper-coatedv lead powder in which co isV plated from a solution upon particles oi'. the improvement which comprisespassing a direct current: through an aqueous acetate containing acetic acid and relatively large proportions of. copper and 'of lead insolutionzin an electrolytic -cell containing a cathode'andmaintainlng the'cathodecurrent density in er that which a. massive de- A whereby the particles and said particles become plated with copper from the electrolyte. I

5. In the manufacture of copper-coateddead powder in which copper is plated from a solution upon particles of lead, the improvement which `comprises passing adirect current through an aqueous acetate electrolyte containing' copper acetate, lead acetate and acetic acid in relatively large proportions in an electrolytic cell containing a cathode and maintaining the cathode current l density in excess of that at which the 'lead deposits in massive format the cathode, whereby the lead deposits as substantially non-adherent iine particles and said particles become plated with copper from the electrolyte.

6. In the manufacture of copper-coated lead powder in which copper is plated from a solution Von particles of lead, the improvement which com- 7. In the manufacture of 4copper-coated lead powder in which copper is plated from solution on particles of lead, the improvement which comprises subjecting to electrolysis in a cell containing a cathode an aqueous acetate electrolyte containing acetic acid relatively large proportions of dissolved copper and lead, said electrolysis being conducted at a cathode current density in excess of that at which the lead deposits at the cathode in massive form, the lead depositing as particles at the cathode and passing through the electrolyte to a lower portion of the cell, the particles of lead -being coated with copper, and removing the resulting Acopper-coated lead powder from the cell.

8. In the manufacture of copper-coated lead powders in which copper is plated from solution onto particles of lead, the improvement which comprises subjecting to electrolysis in a cell having a cathode an aqueous acetate solution containing about 100 grams per liter of lead acetate, 50 grams per liter of copper acetate, and 15 grams per liter ofacetic acid at a cathode current dens'ty of about 17.6 amperes per square foot.

CHARLES LEIGH MANTEIL. 

